The present invention is generally drawn to transmitters using high power amplifiers (HPAs), for example in satellite communications. Satellite communication systems must transmit signals vast distances from earth to satellites in orbit and vice versa. Additionally, satellites have strict power consumption limits that require the communication systems to operate at very high efficiencies of both power use and usage of available communication bandwidth.
Many satellites use HPAs for communication purposes. Typically, HPAs operate most efficiently at (or near) saturation. Unfortunately, operation of HPAs at (or near) saturation can lead to inter-symbol interference (ISI) in output channels.
The output of a transmitter can be seen as a sequence of symbols called a phrase. Each symbol represents a sequence of bits, in the case of 8PSK, each symbol represents 3 bits. A transmitter will output the phrase one symbol at a time during transmission. As a transmitter shifts from one symbol to the next in the phrase, previous and future output symbols may cause interference in the output of the current symbol. This interference in the current symbol caused by past and future symbols is ISI.
To further increase efficiency in satellite communication, a single transmitter may be used to transmit multiple channels. A problem, however, in systems that use a single transmitter to transmit multiple channels, i.e., multi-channel transmitters, is spectral spreading, wherein one channel “bleeds over” into another channel, which is referred to as adjacent channel interference (ACI). This ACI problem worsens as the spacing between channels decreases. Further, if a single HPA for a multi-channel transmitter is driven at or near saturation, the ACI problem compounds and becomes too large to enable a receiver to receive any one channel.
Conventional satellite communication systems with HPAs have been able to address ISI over a single channel. Conventional satellite communication systems with HPAs that have addressed ISI have not been able to additionally correct for ACI. Accordingly, conventional satellite communication systems with HPAs that have addressed ISI are not able to transmit over a plurality of channels.
In essence, conventional satellite communication systems with HPAs are able to: drive a single HPA in or near saturation while efficiently communicating over a single channel; or inefficiently communicate over a plurality of channels without driving a single HPA in or near saturation.
An example conventional transmitter for use in a conventional satellite communication system with an HPA will now be described with reference to FIG. 1.
A conventional multi-channel transmitter is shown in FIG. 1. Transmitter 100 includes a an integer number of signal sources. It should be recognized that a conventional multi-channel transmitter may be designed with a desired number of signal sources. In this example, three signal sources—signal source 102, signal source 104 and signal source 106, are shown, wherein the remaining number (as may be designed) are illustrated by way of dots. Transmitter 100 additionally includes modulator 108, modulator 110, modulator 112, mixer 114, mixer 116 and adder 118.
Signal source 102 generates source signal 124, which is passed to modulator 108. Modulator 108 uses source signal 124 to generate modulated signal 130. Mixer 114 mixes oscillator signal 136 and modulated signal 130 to create channel signal 140, the component of output signal 144 associated with the channel associated with signal source 102. Signal source 104 generates source signal 126, which is passed to modulator 110. Modulator 110 uses source signal 126 to generate modulated signal 132. Although not shown, modulated signal 132 will additionally be mixed with an appropriate oscillator signal to create a distinct channel. A component of output signal 144 will include the mixed modulated signal 132 associated with the channel associated with signal source 104. Signal source 106 generates source signal 128, which is passed to modulator 112. Modulator 112 uses source signal 128 to generate modulated signal 134. Mixer 116 mixes oscillator signal 138 and modulated signal 134 to create channel signal 142, the component of output signal 144 associated with the channel associated with signal source 106. Adder 118 takes modulated signal 132, channel signal 140, and channel signal 142 and adds them to create output signal 144. Further, it should be noted that any further channels included (such as those represented by the dots in the figure) will have generated respective mixed modulated signals that will have been added at adder 118 to be included in output signal 144.
FIG. 1 shows a signal paths with two side sources, signal source 102 and signal source 106, around a main source, signal source 104. As mentioned above, any number of sources may represent a multiple channel transmission system, where each source would be modulated and then mixed before being added to the main output signal. This would be similar to the path of signal source 102 generating source signal 124 to be modulated to modulated signal 130 by modulator 108, which is then mixed with oscillator signal 136 by mixer 114 to create channel signal 140 to be added to output signal 144 by adder 118.
Output signal 144 is then provided to an HPA (not shown) for transmission to a receiver (not shown). Clearly, output signal 144 includes signals from a plurality of channels. As discussed above, in a conventional satellite communication system, the HPA would not be driven at or near saturation in order to transmit output signal 144.
What is needed is an improved communication system employing an HPA that is operable to communicate over a plurality of channels while the HPA is driven at or near saturation.